Destroying evidence and suiciding seem an unlikely combination.i know murder suicide has been ruled out, but it would be a much simpler scenario, what if they had argued, it became violent and accidentally fatal, blood in the tent so it was burnt down to destroy evidence, sparing the precious truck and the body dragged into denser bush where the other suicided?
obviously police have reasons for ruling this out, but it was my first thought when i heard they were missing, that or a familiar person?